For the ignorant ( I mean this in a positive manner) get you a junk m/cycle engine. Practicing loosening,removing, installing and tightening all bolts of all sizes. You will come to understand the meaning of proper torque or tightness of capscrews. If a fastener does not start EASILY you will want to remove it and see if you are cross threading it. This ability comes with repetition and appreciation. There are only 2 to 3 sizes of the majority of fasteners you will normally be disturbing so don't be scared. DO tighten every fastener you loosen. ALL fasteners are important.
When you get into something REALLY important use a torque wrench.